1. Field of the Invention
The present invention relates to a method of manufacturing a printed circuit board.
2. Description of the Background Art
Printed circuit boards are generally produced by the semi-additive method. The semi-additive method includes the steps of forming a thin metal film on an insulating layer by sputtering, forming plating resist in a prescribed pattern on the thin metal film, forming conductive patterns in a region without the plating resist, removing the plating resist, and removing the exposed thin metal film by etching.
In this case, after the exposed thin metal film is removed by etching according to the semi-additive method, a small amount of the residue of the metal film (hereinafter referred to as “residual metal”) may be left on the insulating layer.
FIG. 5 shows schematic views of residual metal present on the insulating layer. As shown in FIG. 5(a), prescribed conductive patterns 3 are formed on the insulating layer 1 with a thin metal film 2 interposed therebetween.
FIG. 5(b) is an enlarged view of the region A between the conductive patterns 3 on the insulating layer 1 in FIG. 5(a).
As shown in FIG. 5(b), a plurality of residual metal pieces 2a are present on the insulating layer 1 between the conductive patterns 3.
When the printed circuit board is used in a high temperature and high humidity environment, a short-circuit may be caused between the conductive patterns in some cases because of ion migration. The residual metal can accelerate the occurrence of ion migration.
Therefore, methods of removing the residual metal by carrying out plasma etching have been proposed (see for example JP 2004-259774 A).
The disclosed conventional plasma etching is however anisotropic etching, and therefore the residual metal pieces 2a on the insulating layer 1 serve as masks against the plasma as shown in FIG. 6, and the region of the insulating layer 1 excluding the region under the residual metal pieces 2a is etched. As a result, the residual metal is not sufficiently removed. Note that in the anisotropic etching, etching is accelerated in the thickness-wise direction of the object.